Rabois was COO at payment startup Square, but Stein and his co-founders decided to sell their company to their former Google boss, Marissa Mayer, instead.

"They were crazy enough to turn me down for Yahoo," Rabois says.

Stein says, "Square and Stamped were both in the local space, and Stamped had built a core technical competency in that area. We loved Keith and the team at Square but Marissa gave us an opportunity to build an office for Yahoo in New York City, which was too exciting an opportunity to pass up (and it let us stay in NYC!)"

Stamped was the first of more than a dozen startups Mayer acquired, including Tumblr and another Rabois-backed company, Qwiki. After two years, Stein left Yahoo to start his next venture and Rabois, now a VC at Khosla Ventures, was one of the first people he called.

Stein's new company is called, "Sup." It's a short video messaging app. You can "Sup" a friend. If a friend accepts your request, it turns on the video camera on their phone and you can see what they're up to for 10 seconds. The current product looks like a game, where you can control friends and tell them to take selfies, move left, right or forward with their cameras to see their view points.

At first glance, it seems like a trivial collection of all the hottest fads in tech startup land. It has a short name like Yo. It does short, ephemeral video messaging like Snapchat. It's social, like everything else. But the long term vision is to let users see what's going on anywhere in the world in real-time by utilizing millions of cameras that aren't in use. Instead of reading tweets about what it's like in Gaza, for example, you could 'Sup' someone who's there to see what's going on, like virtual teleportation.

That long-term vision is what convinced Rabois to invest. It didn't hurt that Stein's pitch also made him "giggle."

During his pitch at Khosla Ventures, Stein says he had a friend strategically planted near the Bryant Park skating rink in Manhattan, ready to dive onto the ice and do something ridiculous the moment Stein sent a "sup" invitation. Stein also demonstrated how taking over someone's point of view could help Khosla spy on other VC firms, which Rabois appreciated.

"They jokingly said, 'You know what, we can get people to hang out in the parking lots of other VC firms, so you can see which entrepreneurs are pulling up to places like Sequoia Capital,'" Rabois says. "We're a competitive group, so it was funny."

They jokingly said, 'You know what, we can get people to hang out in the parking lots of other VC firms, so you can see which entrepreneurs are pulling up to Sequoia Capital.'

Stein admits the long-term vision for Sup might be a bit ahead of its time. He says he likes to pick startup ideas that "skate in front of the puck."

Rabois isn't worried about Stein's ambition, even though startups that are too forward thinking have been known to die.

"I don’t believe startups are ever too early," says Rabois. "Entrepreneurs put the world where they want it to be. That’s the number one job and responsibility for founders — to create product market fit today, not in the future."

Here's how to use Stein's new app.

When you open Sup, you can add friends from your mobile contact list or select a suggested person to video control (including Sup's founders, who are using their current mobile phones to deal with customer support issues via text.)

When you send a Sup request, Sup notifies the friend you want to start a video session with. It pings you when your friend has accepted or declined the request, and then connects your two devices in a FaceTime-like call.

But instead of both people being able to see each other's camera view-point, only the person who requested the conversation can see a video feed. The friend then becomes a camera man, taking directions to turn left or right, forward or backward, however the "Supper" swipes their finger on the screen...

A friend can also instruct the camera person to flip the view and take a selfie, or tell them to make certain faces. The video chat ends after a few seconds, or whenever you hit the "X" in the top left of the screen.

"Sup is unlocking the potential of 500 million live video cameras sitting unused in our pockets every day," says Stein. "Right now, you can use it with your friends. In the future, you'll be able to ask anyone to jump into their eyes, anywhere in the world."